1. Field of the Invention
The present invention concerns a multi-wound metal tube prepared by winding and brazing a hoop material (metal hoop material), as well as a method and an apparatus for preparing the same.
2. Description of the Prior Art
A method of preparing a multi-wound metal tube includes a method of plastically deforming a hoop material (metal hoop material) having a copper brazing material applied over the entire surface thereof into a tubular body by a forming apparatus, melting the brazing material between walls of the multi-wound tubular body by a heating device and then coagulating the molten brazing material by a cooling device to obtain a final product. Means for melting the brazing material of the multi-wound metal tube in this method includes a method of using an electric furnace and a method of ohmic heat generation by electric current supply. The method of using the electric furnace comprises cutting a metal tube formed into a multi-wound wall structure by a forming apparatus into a predetermined length and sending the multi-wound metal tubes each of a predetermined length successively into the electric furnace and melting the brazing material, while the ohmic heat generation by electric current supply comprises supplying electric current to a tubular body by way of two electrodes spaced apart from each other in the longitudinal direction of the tubular body delivered continuously from the forming apparatus, melting the brazing material by the ohmic heat generation of the tubular body and conducting brazing continuously (refer to U.S. Pat. No. 2,746,141 and German Federal Republic Patent Specification No. 813839). However, the prior method for manufacturing the multi-wound metal tube involves the following problems.
That is, abrasion of a forming tool incorporated into the forming apparatus variation of mechanical property or size of an elongate hoop material (usually longer than 1,000 m) wound in a coiled shape sometimes causes gaps between each of multi-wound walls or peeling between inner and outer seam portions, in a wall forming step. If the multi-wound metal tube is in such a state, gaps are formed in some places which hinders adhesion of the brazing layer material between each of the wound walls even after brazing, to provide poor adhesion, as well as adhesion for seamed portions is poor, failing to provide a function as a pipeline and it can not but be discarded as failed products.
FIGS. 20(A) and 20(B) are photographs showing a portion of a cross-sectional structure of a multi-wound metal tube manufactured by an existent method, in which FIG. 20(A) is a photograph showing a portion between walls and FIG. 20(B) is an enlarged photograph for an outer seam portion. As apparent from the photographs of FIGS. 20(A) and (B), in a multi-wound metal tube manufactured by the existent method, gaps are sometimes formed between wound walls in which a brazing layer is not present, to make adhesion poor, and a step is formed to an outer seam portion, which may result in a problem in view of the quality in a case where sealing performance is required between the outer circumferential surface of the tube and a rubber hose or on O-ring when the latter is fitted over the surface of the tube upon use.
Further, in the existent method, since there is no appropriate means for eliminating the gaps between each of the multi-wound walls caused in the forming step, materials have to be selected while considering the mechanical property, for example, spring back of the hoop material, which restricts the range for selecting the materials. Usually, a hoop material having a length of more than 100 m has been shaped as it is without correcting dimensional scattering for each of portions. Further, in view of the gaps between the multi-wound walls caused in the forming step, a great amount of the brazing material is required, to increase the cost for the brazing material such as copper and, in addition, this increases the thickness of the copper layer as the brazing material (refer to photograph in FIG. 20B) and there may be a worry of causing a problem of embrittlement along with increase of the copper layer, which is not preferred.